In the field of electronics, it is occasionally necessary to utilize a device which selectively generates an electrical resistance which is used to load a circuit. As an example, during the testing and troubleshooting of power supplies, a decade box may be connected across the power terminals and, in conjunction with a volt meter and an ammeter, one may ascertain whether or not the power supply is performing at its rated capability. This is achieved by changing the resistance of the decade box, and thus the load on the power supply, while observing current and voltage measurements taken by the ammeter and volt meter. While a decade box can be used to generate a variable resistance, it is a cumbersome device which requires the user to incrementally operate several rotary switches to obtain the desired resistance. Further, a smoothly rising or falling resistance would be almost impossible to duplicate with a decade box because of the necessity to operate several of these rotary switches simultaneously.
Another device for generating a variable resistance is a potentiometer. While this device can be used to generate a variable resistance, it is a mechanical device and is subject to wear and mechanical failure. Further, potentiometers are limited to a single range of resistance, requiring a potentiometer for each desired range of resistance.
Other devices which may be used to generate a variable resistance include digitally controlled audio attenuators, two types of which are manufactured by National Semiconductor and DBX of New York. The National device can be incremented in 3 db steps, which attenuates voltage or current by a factor 0.5, while the DBX device is capable of attenuating in 1 db steps. Problems with these attenuators, however, are that even the smallest db steps do not provide the resolution necessary to mimic a smoothly rising or falling resistance. Further, they cannot duplicate a linear resistive output because of logarithmic characteristics associated with attenuators. Still further, these attenuators exhibit distortions which may be tolerable at audio frequencies but which make them unsuitable to duplicate precise resistances, particularly below 100 ohms.
Yet another device that can generally provide a variable resistive output is an optical light emitting coupler manufactured by Clairex Company. It consists of a cadmium sulfide photocell and a light source mounted in close proximity to each other within a tubular housing. In operation, the photocell changes its resistance responsive to the degree of illumination provided by the light source. The major problem with this device is that the resistive output is not linear with respect to a variable voltage applied to the light source.
It is therefore an object of this invention to provide a digitally controlled variable resistance generator which has no mechanical parts and which is further capable of providing a multitude of linear ranges of resistance.